There are several applications, including military ones, associated with infrared imaging in the 3-5 .mu.m and 8-12 .mu.m wavelength ranges (where atmospheric absorption is minimal). These have motivated substantial research efforts to develop intrinsic semiconductor detectors capable of operation at these wavelengths. Intrinsic detectors are those wherein absorption is based on excitation of carriers from valence bands to conduction bands. Although a number of materials appear suitable for fabricating imaging arrays for the 3-5 .mu.m range, no bulk III-V materials have bandgap values small enough for cooled (e.g , 77.degree. K.) (low noise) detector operation in the important 8-12 .mu.m range. As a result, it has been necessary to consider a number of II-VI and IV-VI materials as potential candidates for 8-12 .mu.m applications.
Most of the research on these applications in recent years has concentrated on the II-VI alloy Hg.sub.1-x Cd.sub.x Te. This alloy has a range of bandgaps (at 0.degree. K.) which extends from Eg.about.1.6 eV (x=1.0) to Eg=0.0 eV (x.about.0.16), and the alloy with x=0.205 has the required bandgap for 77.degree. K. detector operation in the 8-12 .mu.m atmospheric window. See, e.g., Semiconductors and Semimetals, Willardson and Beer, Ed, Vol. 18, "Mercury Cadmium Telluride", Academic Press (1981). However, the HgCdTe alloys which are Hg rich have a number of metallurgical and device related problems which will make satisfactory 8-12 .mu.m imaging arrays in these materials difficult if not impossible to achieve. Some of these problems are listed below:
(1) HgCdTe is mechanically brittle. PA1 (2) The hole concentrations in p-type HgCdTe are difficult to control; n-type "doping" is typically produced by unannealed implantation damage. PA1 (3) Above.about.100.degree. C., Hg outdiffusion becomes a problem (causing n-type material to convert to p-type). This suggests long term stability problems, and eliminates the possibility of high temperature device processing of HgCdTe wafers. PA1 (4) The Hg.sub.1-x Cd.sub.x Te bandgap and the associated IR detector cutoff wavelength vary rapidly with alloy composition. As a result, the composition must be held to within about .+-.0.3% across the entire wafer to avoid large, lateral non-uniformities in detector performance (i.e., poor device yield). PA1 (5) The extremely small electron effective mass (m.sub.e *) in the Hg rich alloys causes (conduction) band to (valence) band tunneling to be significant. This causes an experimentally observed noise component that cannot be reduced by cooling the detector. In addition, tunneling reduces the photodiode dynamic resistance at zero bias (R.sub.o) defined by R.sub.o.sup.-1.tbd. (dI/dV).vertline..sub.v=o. This may prevent the successful operation of 8-12 .mu.m detector arrays which make use of charge-coupled device (CCD) readouts, since CCD's need a certain minimum R.sub.o. PA1 the strain resulting from the mismatch of lattice constants and said ratio of layer thicknesses being effective to narrow the bandgaps of said first set of III-V layers, thereby changing the intrinsic radiation absorption characteristics of said layers when cooled to include wavelengths in the 8-12 .mu.m region which are larger than those to which said individual layers would be responsive when cooled and in bulk form.